Asymmetric digital subscriber line (ADSL) systems enable data to be transmitted over a pair of metallic twisted pair (usually copper) wires to customer premises. It is thought that the maximum transmission performance that is likely to be obtained with modern variants of ADSL is a download data rate of 24 Mbps and an upload speed of about 3 Mbps. Such data rates are dependent on the length, insulation dielectric and gauge of the metallic twisted pair from the customer premises to the telephone exchange and thus many customers will receive services at significantly lower data rates.
To improve data rates optical fiber has been installed into the access network. The greatest data rates are likely to be provided using fiber to the premises (FTTP) networks, such as passive optical networks (PONs), but there is a significant cost involved in providing fiber all the way to customer premises. Fiber to the cabinet (FTTCab) networks are known to provide an attractive solution to providing customers with high data rate services without requiring as much investment as FTTP networks. Typically in FTTCab networks, very high bit-rate digital subscriber line (VDSL) systems are used to provide data rates of 40 Mbps and higher, for both upload and download on the metallic twisted pair cables. It is believed that improvements to VDSL systems, particularly vectoring may provide data rates in excess of 100 Mbps in the future.
DSL systems work by utilizing the frequencies above those which are used by the conventional telephony signals. In particular, VDSL2 defines multiple frequency windows for downstream and upstream data, according to which internationally standardized band plan is set. Each of these windows comprises a number of data transmission carriers which normally have a 4.3125 kHz frequency separation. Each of these carriers will transmit one or more symbols with each of these symbols being used to transmit up to 15 bits of data. During a training process the insertion loss and noise level are determined for each of the carriers such that the signal to noise ratio (SNR) for each carrier can be determined. The training process determines the capacity of the upstream and downstream links in accordance with the SNRs of each of the carriers.
FIG. 1 shows a schematic depiction of a FTTCab network in which a telephone exchange 100 is connected to a plurality of cabinets 120 by optical fiber cables 110. These cabinets 120 comprise the opto-electronic equipment necessary to send data to the customer premises 200 over a twisted metallic (usually copper) cable 130. Typically, a customer will use the communications network to access the internet and to access data such as video on demand services. Alternatively, the network may be used to transmit television channels which might conventionally be transmitted using a terrestrial or satellite transmission channel. One of the limitations of DSL systems is that the data signals are attenuated as they are transmitted across the metallic twisted pair cables. The maximum design range of the VDSL technology from the cabinet to the customer premises is typically 1.5-2 km, but often some outlying customer premises are located at distances much greater than the maximum design range. Beyond this maximum range then it is not usually possible to deliver a reliable data service of greater than 2 Mbps (this has been set by Ofcom as being the minimum data rate for all UK customers by the end of 2015). However, there are a significant number of customers whose properties are significantly further from the cabinet than this maximum range. A solution to this problem would be to deploy fiber further into the periphery of the access network but this is an expensive solution for a small number of customer premises.
Another solution is to use a VDSL amplifier to boost the signals in one or more of the frequency bands assigned to downloading (and to also uploading). Such amplifiers can increase the maximum range of a VDSL system to typically 3-4 km. An example of such an amplifier is the VBA (VDSL Broadband Amplifier) provided by Actelis Networks. FIG. 2 shows a schematic depiction of the network described above with reference to FIG. 1 in which a VDSL amplifier 150 has been placed in line with each of the metallic twisted pair cables that form the D-side or Distribution Side access network 130.
When a VDSL cabinet 120 is installed, it is necessary to determine a parameter referred to as the CAL, or Cabinet Assigned Loss. This parameter describes the electrical length of the cables between the exchange and the cabinet (this portion of the access network is referred to as the Exchange-side (or E-side) network). The CAL value will be entered into the DSLAM housed in the cabinet and this value will be used for determining the power spectral density (PSD) of the VDSL signals used when transmitting data to the customer premises. Furthermore, it is necessary that the CAL parameter value be entered into a VDSL amplifier to ensure its proper operation within the limits of any defined spectrum management rules such as the UK ANFP (NICC ND:1602). In the UK ANFP, the CAL parameter is a whole number value in the range 0-52 dB, and each VDSL2 cabinet or DSLAM has such a value set which is a function of the E-side length and cable routing from the serving exchange to the cabinet.